Turbofan engines have proven to be desirable for commercial aircraft, which travel at subsonic speeds. A turbofan engine transfers energy from a jet stream of gas to a turbine shaft; the shaft turns a ducted fan which moves a mass of cold air, providing a thrust that propels the aircraft. The thrust is varied by changing the pitch of the fan blades. During takeoff, when high thrust is required, the pitch of the blades is adjusted to produce maximum engine torque. At cruise, when lower speeds are required, the blade pitch is adjusted to reduce engine torque. During landings, the pitch is adjusted to produce a reverse thrust, which brakes the aircraft. Among the advantages offered by the turbofan engine is low noise level, since jet velocities are reduced. Further, the reverse thrust capability permits the elimination of a thrust reversal mechanism. This offsets to a certain degree the weight and complexity introduced by the control system that varies blade pitch. Additionally, fuel efficiency of the turbofan engine is increased since blade pitch can be varied to cater to ever-changing flight conditions. It is claimed that fuel consumption can be increased by as much as twelve percent by adjusting the pitch of the blades.
The blade pitch is adjusted by a mechanical blade pitch actuator, which can be mechanically, hydraulically or pneumatically driven. In a typical mechanical blade pitch actuator, the difference of rotary displacement between two shafts, a fan drive shaft and a control shaft, is used to adjust the pitch of the blades. The control shaft is selectively rotated at a rate different than that of the fan shaft until the desired degree of blade angle is realized. For instance, when the control shaft is rotated at an angular rate greater than that of the fan shaft, the blade pitch is increased. At a slower rate, the blade pitch is decreased. When the control shaft and fan shaft are rotated at the same angular rate, the blade pitch is maintained.
The blade pitch actuator should include apparatus to measure the blade pitch as the fan blades are being adjusted. The measurement should be precise, and error due to vibrations that occur when the engine is running should be minimized. Further, the actuator should be able to measure blade pitch with constant precision regardless of whether the engine is running or stopped.